Editing Steel (section)

===Properties===
[[File:Steel_Fe-C_phase_diagram-en.png|thumb|upright=1.5|Fe-C phase diagram for carbon steels, showing the A<sub>0</sub>, A<sub>1</sub>, A<sub>2</sub> and A<sub>3</sub> critical temperatures for heat treatments]]
The [[density]] of steel varies based on the alloying constituents but usually ranges between {{convert|7750|and|8050|kg/m3|lb/ft3|abbr=on}}, or {{convert|7.75|and|8.05|g/cm3|oz/cuin|abbr=on}}.<ref>{{cite web |last=Elert |first=Glenn |title=Density of Steel |url= http://hypertextbook.com/facts/2004/KarenSutherland.shtml |access-date=23 April 2009}}</ref>

Even in a narrow range of concentrations of mixtures of carbon and iron that make steel, several different metallurgical structures, with very different properties can form. Understanding such properties is essential to making quality steel. At [[room temperature]], the most stable form of pure iron is the [[body-centred cubic]] (BCC) structure called alpha iron or α-iron. It is a fairly soft metal that can dissolve only a small concentration of carbon, no more than 0.005% at {{Convert|0|C|F|abbr=on}} and 0.021 wt% at {{convert|723|C|F|abbr=on}}. The inclusion of carbon in alpha iron is called [[Allotropes of iron|ferrite]]. At 910&nbsp;°C, pure iron transforms into a [[face-centred cubic]] (FCC) structure, called gamma iron or γ-iron. The inclusion of carbon in gamma iron is called austenite. The more open FCC structure of austenite can dissolve considerably more carbon, as much as 2.1%,<ref>Sources differ on this value so it has been rounded to 2.1%, however the exact value is rather academic because plain-carbon steel is very rarely made with this level of carbon. See:
* {{harvnb|Smith|Hashemi|2006|p=363}}—2.08%.
* {{harvnb|Degarmo|Black|Kohser|2003|p=75}}—2.11%.
* {{harvnb|Ashby|Jones|1992}}—2.14%.</ref> (38 times that of ferrite) carbon at {{convert|1148|C|F|abbr=on}}, which reflects the upper carbon content of steel, beyond which is cast iron.<ref>{{harvnb|Smith|Hashemi|2006|p=363}}.</ref> When carbon moves out of solution with iron, it forms a very hard, but brittle material called cementite (Fe<sub>3</sub>C).{{Cn|date=January 2024}}

When steels with exactly 0.8% carbon (known as a eutectoid steel), are cooled, the [[austenitic]] phase (FCC) of the mixture attempts to revert to the ferrite phase (BCC). The carbon no longer fits within the FCC austenite structure, resulting in an excess of carbon. One way for carbon to leave the austenite is for it to [[precipitate]] out of solution as [[cementite]], leaving behind a surrounding phase of BCC iron called ferrite with a small percentage of carbon in solution. The two, cementite and ferrite, precipitate simultaneously producing a layered structure called [[pearlite]], named for its resemblance to [[mother of pearl]]. In a hypereutectoid composition (greater than 0.8% carbon), the carbon will first precipitate out as large inclusions of cementite at the austenite [[grain boundaries]] until the percentage of carbon in the [[Grain (metal)|grains]] has decreased to the eutectoid composition (0.8% carbon), at which point the pearlite structure forms. For steels that have less than 0.8% carbon (hypoeutectoid), ferrite will first form within the grains until the remaining composition rises to 0.8% of carbon, at which point the pearlite structure will form. No large inclusions of cementite will form at the boundaries in hypoeutectoid steel.<ref>{{harvnb|Smith|Hashemi|2006|pp=365–372}}.</ref> The above assumes that the cooling process is very slow, allowing enough time for the carbon to migrate.{{Cn|date=January 2024}}

As the rate of cooling is increased the carbon will have less time to migrate to form carbide at the grain boundaries but will have increasingly large amounts of pearlite of a finer and finer structure within the grains; hence the carbide is more widely dispersed and acts to prevent slip of defects within those grains, resulting in hardening of the steel. At the very high cooling rates produced by quenching, the carbon has no time to migrate but is locked within the face-centred austenite and forms [[martensite]]. Martensite is a highly strained and stressed, supersaturated form of carbon and iron and is exceedingly hard but brittle. Depending on the carbon content, the martensitic phase takes different forms. Below 0.2% carbon, it takes on a ferrite BCC crystal form, but at higher carbon content it takes a [[body-centred tetragonal]] (BCT) structure. There is no thermal [[activation energy]] for the transformation from austenite to martensite.{{clarify|date=April 2016}} There is no compositional change, so the atoms generally retain their same neighbours.<ref name="smith&hashemi">{{Harvnb|Smith|Hashemi|2006|pp=373–378}}.</ref>

Martensite has a lower density (it expands during the cooling) than does austenite, so that the transformation between them results in a change of volume. In this case, expansion occurs. Internal stresses from this expansion generally take the form of [[physical compression|compression]] on the crystals of martensite and [[tension (mechanics)|tension]] on the remaining ferrite, with a fair amount of [[shear stress|shear]] on both constituents. If quenching is done improperly, the internal stresses can cause a part to shatter as it cools. At the very least, they cause internal [[work hardening]] and other microscopic imperfections. It is common for quench cracks to form when steel is water quenched, although they may not always be visible.<ref>{{cite web |title=Quench hardening of steel |url= http://steel.keytometals.com/default.aspx?ID=CheckArticle&NM=12 |access-date=19 July 2009 |work=keytometals.com |url-status=dead |archive-url= https://web.archive.org/web/20090217103241/http://steel.keytometals.com/default.aspx?ID=CheckArticle&NM=12 |archive-date=17 February 2009}}</ref>